A brush seal of the prior art will now be described with reference to FIGS. 1A, 1B, 1C and 2 of the accompanying diagrammatic drawings, in which,
FIG. 1A is a view of the prior art brush seal taken along its axis of rotation,
FIG. 1B is a cross section of the brush seal of FIG. 1A taken along line X--X, and
FIGS. 1C and 2 are views of FIG. 1B showing the brush seal undergoing different types of distortion.
In FIG. 1A a large number of tightly packed bristles 1 are shown secured between a pair of annular sideplates 2,3 which are attached to a fixed housing 4. The free ends 5 of the bristles 1 are in rubbing contact with the surface of a shaft 6 which rotates relative to the housing 4.
As a result of the tight packing of the bristles and their rubbing contact with the shaft fluid leakage through the seal is low.
If the shaft 6 moves relative to the housing 4 the bristles 1 bend elastically in the plane of the seal to accommodate this movement returning to their original positions as the shaft moves back. Clearly the longer the bristles 1 are the greater the degree of movement of the shaft 6 so they will be able to accommodate without any of the bristles losing contact with the surface of the shaft 6 and reducing the quality of the seal.
If there is a pressure difference across the seal the pressure will exert a force on the bristles 1 which will cause them to bend along the axis of the shaft 6 towards the low pressure side as shown in figure 1B.
If the differential pressure across the seal is high enough the bristles 1 will bend so far that they lose contact with the surface of the shaft 6 and the seal will fail. The longer the bristles 1 are the larger the pressure forces on them and the degree of bending they suffer will be for a given differential pressure. As a result, the longer the bristles are the lower the pressure at which the seal will fail.
In order to overcome the problem that longer bristles for increased tolerance of movement will reduce the maximum differential pressure the seal can withstand annular sideplates 2,3 are arranged to have different inner radii.
The annular sideplate 2 on the high pressure side of the seal has a larger inner radius than the annular sideplate 3 on the low pressure side of the seal. As a result the bending of the bristles 1 due to the differential pressure across the seal is reduced because the bristles 1 are supported by the sideplate 3, and the ability of the bristles 1 to accommodate movement of the shaft 6 relative to the housing 4 is unchanged because the bristles 1 can bend in the plane of the seal from the edge of the sideplate 2.
Unfortunately there is still a problem with this arrangement. When a differential pressure is applied across the seal the bristles 1 bend towards the lower pressure side of the seal and as a result they protrude between the sideplate 3 and the shaft 6. As a result movement of the shaft 6 towards the housing 4 can result in some of the bristles 1 being pinched between the shaft 6 and the sideplate and bending perpendicular to the plane of the seal instead of in the plane of the seal, as shown in figure 1C. This can cause these bristles 1 to bend so sharply that they snap or plastically deform, degrading the quality of the seal.
The basis of this invention is the realisation that a bristle of a brush seal of the type shown in figure IA bends as cantilever which is supported where the bristle is held between the two sideplates and where it bends over the end of the sideplate with the smaller inner diameter.
For example, looking at FIG. 2 a seal is formed between a housing 4 and a shaft 6 by a plurality of bristles 1 secured between a pair of sideplates 2,3 as before. In order to reduce the degree of pressure induced bending of the bristles 1 when a differential pressure is applied across the seal the sideplate 3 on the low pressure side of the seal has a smaller internal diameter than the sideplate 2 on the high pressure side.
It has been found that when a differential pressure is applied across the seal the bristles bend as a cantilever. That is the bristles 1 bend from the edge of the sideplate 2 with the larger inner radius towards the high pressure side of the seal, then curve back towards the low pressure side of the seal and are supported by the sideplate 3 having the lower inner radius at its inner edge 7 and then project towards the low pressure side of the seal between the sideplate 3 and the shaft 6. As a result the bristles 1 are not in contact with the sealplate 3 in the region 8 between the inner edge of the sealplate 2 and the inner edge 7 of the sealplate 3.